Bacteria are single-celled organisms that are found almost everywhere, exist in large numbers and are capable of dividing and multiplying rapidly. Most bacteria are harmless, but there are three harmful groups; namely: cocci, spirilla, and bacilla. The cocci bacteria are round cells, the spirilla bacteria are coil-shaped cells, and the bacilli bacteria are rod-shaped. The harmful bacteria cause diseases such as tetanus and typhoid.
Viruses can only live and multiply by taking over other cells, i.e. they cannot survive on their own. Viruses cause diseases such as colds, flue, mumps and AIDS.
Fungal spores and tiny organisms called protozoa can cause illness.
Sterilisation is an act or process that destroys or eliminates all form of life, especially micro-organisms. During the process of plasma sterilisation, active agents are produced. These active agents are high intensity ultraviolet photons and free radicals, which are atoms or assemblies of atoms with chemically unpaired electrons. An attractive feature of plasma sterilisation is that it is possible to achieve sterilisation at relatively low temperatures, such as body temperature. Plasma sterilisation also has the benefit that it is safe to the operator and the patient.
Low temperature atmospheric pressure plasmas may be used to replace conventional sterilisation methods and offer clear advantage over existing means of sterilisation in terms of their non-toxic nature, instant treatment effects, and the ability to produce the plasma at a range of energy levels and in a range of different forms. In a room temperature environment, plasma is usually supported by electro-magnetic (EM) fields. Light electrons absorb energy from an electric field and transfer part of this energy to heavy particles in the plasma. If electrons are not given sufficient opportunity to transfer their energy, heavier plasma components remain at much lower temperatures than the electrons. Such plasmas are called non-thermal plasma and their gas temperatures can be as low as room temperature.
Active plasma particles (electrons, ions, radicals, and other chemically active species) and UV radiation may be used to disinfect and sterilise living tissue, biological inserts placed inside living tissue, external surfaces, or surgical instruments. The closer the plasma source is located with respect to the living tissue (or other surfaces) and the higher the electric field in the plasma, the higher the intensity and efficacy of the non-thermal plasma sterilisation treatment process.
For practical use inside or on the surface of the body, i.e. for wound sterilisation to kill bacteria within the wound or bacteria that resides on the surface of the wound, the destruction or reduction of bacteria contained within natural orifices inside the body, to kill bacteria contained on inserts placed inside the human body (and that manifested within biological tissue in the vicinity of the insert), or to kill bacteria that may exist on the skin before opening up the patient (and to re-sterilise prior to closure) and other surfaces that are required to be sterilised where it is undesirable for the temperature to rise excessively, i.e. for beds, curtains, instruments, pillows and certain plastics, the temperature at the surface or at the treatment site (the biological tissue or environment) produced by the plasma should not exceed normal human body temperature. It may be desirable to consider the maximum temperature at a surface produced by a plasma jet to be limited to a maximum of 10° C. above room temperature, i.e. Tr≦Tt≦Tr+10, where Tr is room temperature (° C.), and Tt is the treatment temperature (° C.). A nominal temperature for non-thermal plasma is 37° C.
Although for some applications it is desirable to operate within these boundaries, the invention described herein may not be limited by such. For example, it may be desirable to increase the temperature well above body temperature when considering the sterilisation of hospital floors, hospital beds or other general materials within a hospital environment where temperatures in excess of body temperature can be tolerated.
The length of the plasma and the temperature produced at a surface may be found using the energy balance, i.e. electron-induced heating of heavy particles versus energy losses by thermal conduction. The length of a plasma jet may be calculated as follows:
                              L          =                                                                      m                  i                                ⁢                3                ⁢                                  K                  i                                ⁢                Δ                ⁢                                                                  ⁢                T                                                              m                  e                                ⁢                                  v                  i                                ⁢                                  N                  D                                ⁢                                  kT                  e                                                                    ,                    1      
where mi is atomic mass, Ki is thermal conductivity, vi is effective electron-atom collision frequency, ND is electron density, ΔT is temperature change and Te, is energy level. Tables 1 and 2 list the parameters for calculating plasma length in certain gases. Table 3 lists typical plasma lengths at room temperature for those gases.
TABLE 1Parameters for calculation of plasma length at atemperature of 300 K and pressure of 1 Torr (133.3 Pa), andwhere u is an atomic mass unit (= 1.66 × 10−27 kg).ArCO2HeN2Ki (Wm−1K−1)16.214.5146.1024.3vi (s−1)6.3117 × 1073.22 × 1092.27 × 1081.497 × 108mi (kg)39.948u44.01u4.002602u14.0067u
TABLE 2Other parameters used in the calculationParameterValueND3.22 × 1022m−3me9.109 × 10−31kgk1.380622 × 10−23Te3eVΔT10K
TABLE 3Calculated plasma length at a temperature of 300 Kand pressure of 1 Torr (133.3 Pa)ArCO2HeN2Plasma Length5.35 × 10−47.438 × 10−58.478 × 10−44.337 × 10−4(m)
The non-thermal plasma can be used to create highly reactive molecules called free radicals that can be used to break down contaminants. Free radicals are atoms or molecules that have unpaired electrons.
Non-thermal plasma cells may use a high voltage electric field to create large quantities of highly reactive free radicals. The free radicals may be used to react with and break up hazardous organic chemicals to convert them into non-hazardous substances, such as carbon dioxide or water.
Ultraviolet photons in the plasma affect bacteria cells by inducing the formation of thymine dimers in the DNA. This inhibits the ability of the bacteria to replicate properly. This effect may be particularly useful in the application of treating sexually transmitted diseases where it may be desirable to reduce the level of bacteria, but not totally destroy it, i.e. so as not to destroy the body's natural flora.
It is also recognised that reactive species created in the plasma play an important role in sterilisation. In particular, discharges containing oxygen may have a strong germicidal effect. For example, plasma typically contains charged electrons and ions as well as chemically active species, such as ozone, nitrous oxides, and hydroxyl radicals. As an example of a clinical effect that may be produced using these systems, nitric oxide plasma may be produced using a helium gas and air, whereby the helium helps the plasma to form efficiently from air at low energies; if this plasma could be inserted into the body then it could be used to help fight infection and inflammation—this could be particularly useful for minimally invasive or keyhole applications, e.g. treatment of sexually transmitted diseases or body insert sterilisation. Hydroxyl radicals that may be produced from plasma are another useful source as they are far more effective at oxidizing pollutants in the air than ozone and are several times more germicidal and fungicidal than chlorine, which makes them very interesting in terms of destroying mould, bacteria and viruses.
It has also been suggested that charged particles may play a significant role in rupturing the outer membrane of the bacteria cells. Electrostatic force caused by charge accumulation on the outer surface of the cells' membrane can overcome the tensile strength of the membrane and therefore cause it to rupture. This process is more likely to occur for gram-negative bacteria, which possess irregular surfaces.